Internal combustion engines are controlled to maintain a desired air-to-fuel ratio (AFR) in the combustion chamber to reduce emissions. Fuel is delivered via electronically controlled fuel injectors which may be coupled inside each engine cylinder or located in intake ports of the cylinders, for example. However, not all injected fuel enters the combustion chamber. Rather, some fuel is stored in the intake manifold of the engine resulting in a phenomenon commonly known as “wall wetting”. For example, in an engine configured with port fuel injection, fuel is injected into an intake port, on the back of a closed intake valve during a non-inducting stroke of the cylinder. The injected fuel quickly vaporizes due to the heat from the valve and mixes with the intake air, and the air-fuel mixture is then inducted into the cylinder during an intake stroke. However, the vaporization of the fuel in the intake port is a function of the wall temperature and manifold pressure. Consequently, based on the engine operating conditions, the injected fuel will impact the rear of the wall and some part of it will cause wall wetting or puddling of fuel in the port. Some portion of the liquid phase fuel may remain in the port throughout the cycle resulting in a net delay of the fuel injected.
During steady state operation of the engine, the fuel film is in quasi-equilibrium wherein the amount of fuel added to the film each cycle by the fuel injection is equal to the fuel removed by vaporization and liquid film flow. However, if an engine throttle transient occurs, the air flow and fuel injector response may be very fast (e.g., limited only by manifold air dynamics), while the net fuel flow to the engine cylinder may be limited by changes in fuel film properties. The delay of fuel in the intake port can result in an AFR excursion during a throttle transient. Further, the issue may be exacerbated in engines having cylinders that can be selectively deactivated.
Various approaches have been developed for taking into account the fuel puddles in the intake manifold in controlling engine air fuel ratio during steady-state and transient engine operation. One example attempt is shown by Song et al. in U.S. Pat. No. 7,111,593. Therein, transient fuel wall wetting characteristics of an operating engine are determined while accounting for cylinder valve deactivation. In particular, fuel vaporization effects from fuel vapors leaving the fuel puddles of a deactivated cylinder and migrating to active cylinders are considered when calculating the fueling compensation for the active cylinders.
However, the inventors herein have recognized potential issues with such systems. The inducted air-fuel ratio of the active cylinders may incur fluctuations even with the adjustments of Song. As an example, the rate of evaporation of fuel from the puddle of a cylinder may vary based on whether the given cylinder fired and inducted on the last event. If the cylinder did not induct and fire, the number of events elapsed since the last firing event in the given cylinder may also affect the rate of evaporation of fuel from that cylinder's puddle. Further still, the vapor build-up in the port may be affected by the vapor pressure relative to saturation vapor pressure. Specifically, if the cylinder is deactivated for an extended period, all the puddle or film mass may not vaporize. Instead, the vapor build-up in the intake runner of the deactivated cylinder may quickly reach the saturation vapor pressure limit. Thereafter, the vapor pressure build-up may be limited. As another example, any perturbations in manifold pressure can cause the vapor to escape into the engine's intake manifold and cause additional AFR fluctuations.
In one example, the issues described above may be addressed by a method for an engine, comprising: adjusting a fuel injection responsive to reaching a vapor saturation state in a port of a deactivated cylinder of the engine. In this way, fuel dynamics may be determined more accurately.
As one example, an engine may be configured with a variable displacement enabled via selectively deactivatable engine cylinders. Based on the torque demand, the engine may be operated with a different induction ratio, and accordingly, a cylinder may be skipped or fired on each event. For each cylinder, an engine controller may track the estimated fuel puddle mass and fuel vapor content (e.g., the amount of fuel present in liquid phase relative to vapor phase) using calibrated gains and time constants. The gains and time constants may be calibrated via an X-Tau model as a function of engine operating conditions including manifold pressure, engine speed, mass of injected fuel, and engine temperature. The model may assume that metered fuel is proportional to airflow and that a defined percentage of this fuel impacts the existing puddle and forms a liquid film. A rate of evaporation of fuel from this liquid film is determined as a function of the film thickness or size using the X-Tau model. For a deactivated cylinder, with intake and exhaust valves deactivated, a slower evaporation rate occurs due to lower air flow in the runner of the deactivated cylinder. Thus for each skipped cylinder event, a different time constant is applied as compared to an active cylinder. Further, based on the number of skipped events for a cylinder, it may be determined if the fuel vapor pressure has reached a saturation limit (such as when the fuel vapor content reaches a saturation vapor pressure). The saturation pressure is also affected by the port temperature and the manifold pressure. As such, once the saturation limit is reached, further evaporation of fuel from the port may be limited. Therefore, once the saturation limit is reached, the puddle mass and vapor content for the deactivated cylinder may be clipped. For example, no further change in the puddle mass and vapor content may be registered and the last estimated value of puddle fuel mass and vapor content may be maintained until the cylinder inducts upon reactivation. When the deactivated cylinder is reactivated, fueling is resumed in the cylinder as a function of the clipped values of puddle mass and vapor content. For example, fueling is adjusted to compensate for the amount of fuel vapor pressure resulting from the clipped values of puddle mass and vapor content. At the same time, the fuel puddle mass and vapor content in remaining active cylinders may continue to be estimated based on their vapor pressure, independent of the calculations in the deactivated cylinder(s). Thus in the active cylinders, cylinder fueling may continue to be adjusted to account for wall wetting effects of the fuel puddle.
In this way, by adjusting fuel puddle dynamics of a cylinder based on its induction state, deactivated cylinder relative to an active cylinder, transient fuel compensation can be improved. The technical effect of applying different time constants and gains to account for differing rates of evaporation of fuel from active versus skipped cylinders, a fuel puddle volume can be more reliably learned. By clipping the fuel puddle estimate when the vapor pressure at the puddle reaches a saturation vapor pressure limit, cylinder fueling errors are reduced, particularly when a deactivated cylinder resumes fueling. As a result, a more accurate air-fuel ratio control is provided with fewer AFR perturbations. By tracking and updating vapor content and puddle fuel mass at each skipped cylinder event, it may be possible to provide more accurate fueling to cylinders upon reactivation. Overall, the fuel economy of a variable displacement engine may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.